The present invention relates to a method of and apparatus for reheating cryogenic fluids by heat exchange with heat-carrying fluids having a freezing point which exceeds the temperature of the cryogenic fluid prior to its final reheating.
It is applicable in particular to the reheating of liquified natural gas by means of water available in great quantity such as natural water sources for instance, rivers, lakes and oceans.
It is an object of the invention to be able to make use of water or another fluid at a comparatively low temperature whilst avoiding or minimising any risk of freezing the heat-carrying fluid.
A variety of solutions has already been proposed, none of which however have enabled this result to be obtained.
For example, French Patent Specification No. 70 26,212 discloses a process for reheating natural gas by exchange in counterflow in a plurality of vertical tubes arranged in parallel relationship, the natural gas always flowing in an upward direction within the tubes and the heat-carrying water trickling naturally by force of gravity along the outside of these tubes which are provided with longitudinal fins. In order that the heat exchange may be optimised, that is to say to maximise the heat flow whilst preventing freezing of the water on the outer circumference of the tubes, an ever diminishing internal tube cross-section is provided for passage of the natural gas which results in successive increases in the speed of the natural gas flowing in the tubes. These successive reductions in the passage cross-section were established by insertion of an internal lining formed by a tapered tube of varying cross-section, but the technology for achieving this is rather complex and consequently the cost is high if not prohibitive for most purposes.
Japanese Pat. No. 54 7403 discloses the reheating of natural gas by initial exchange in co-directional flow between the natural gas flowing upwards from below in a tubular cluster and the water flowing upwards from below in a shell or casing in a forced flow, then by an exchange in counterflow between the gas flowing downwards from above in another cluster or nest of tubes and the water flowing upwards from below in the corresponding casing. This procedure is also quite complex and leads to considerable deterioration of the casings in particular, in the case of accidental freezing of the reheating water.
Furthermore Japanese Pat. No. 52 144, 006 discloses a reheating plant comprising a first section for exchange in counterflow between the natural gas flowing upwards from below in a first plurality of tubes and the water trickling naturally on the outside, then a second section equally for exchange in counterflow, the natural gas flowing upwards from below in a second plurality of tubes and the water trickling naturally on the outside, with the feature that the second plurality of tubes opposes a smaller passage cross-section to the natural gas than the first plurality. This system also fails to accomplish the object of the present invention.